How Folsom Lake is using weather forecasts to manage water supply amid climate change

An atmospheric river dumped more than five inches of rain on Sacramento on Oct. 24, breaking a 24-hour rainfall record set in 1880. A week later, Folsom Lake was 16 feet deeper and 90,000 acre-feet fuller — a significant boost in supply for the region’s primary surface reservoir after one of the worst drought years ever.

Just a decade ago, water managers may not have been allowed to hold on to all that water in Folsom. But thanks to Rep. Doris Matsui and investments by local, state and federal officials over the past 10 years, Folsom has $1 billion in new hardware and rules that allow us to store the equivalent of up to two more storms of that size — enough to serve 400,000 homes for a year.

Most of that investment paid for a giant new spillway at Folsom, which helps release more water quickly. But it also paid for a new reservoir operations manual that incorporates the principles of forecast-informed reservoir operations. As the term suggests, decisions to release water from the reservoir are now guided more closely by weather forecasts, using real-time information to increase our region’s resilience in the face of climate realities.

Historically, Folsom Lake managers have been required to maintain up to 670,000 acre-feet of empty volume in the reservoir all winter long and until the end of April to accommodate any floods. That’s two-thirds of the reservoir’s capacity.

If a storm came in February and filled some of that volume, reservoir managers were required to dump enough water to restore the same amount of capacity — even during the worst droughts. If that ended up being winter’s final storm, the reservoir would sit there like a half-empty bathtub even as the calendar marched into summer’s hot, dry months.

Today advances in weather forecasting provide much more certainty about when and where a big storm will hit, and the investments made enable us to be more responsive to a changing climate, providing more resilience in the face of uncertainty.

If the five-day forecast suggests a flood risk, reservoir managers can start to empty water to make room for more runoff. If the forecast shows a storm won’t hit the watershed, they can release less water — or even none. Forecast-informed reservoir operations also allow water managers to take other factors into account, like the wetness of the snowpack. And thanks to the upgraded spillway, operators can wait longer before triggering releases to clear the way for an incoming storm.

It seems incredible that this hasn’t been standard practice all along. But remember, most of our reservoirs were built 60 years ago, when forecasts provided only 24 hours’ notice of a storm. That was never enough time to prepare for a flood.

In addition to allowing the Sacramento region to start summer with more water behind the dam, forecast-informed operations integrate with our groundwater system, another key to adapting water supplies to climate change. Storm water releases eventually could refill groundwater aquifers as another buffer against drought, for example, using the Folsom South Canal, which begins at Lake Natoma, .

Adopting forecast-informed operations at Folsom Lake required painstaking negotiations with the many agencies that have a stake there as well as years of engineering research. It also required that decades of deeply entrenched policy be eased at the federal level.

The work isn’t done. The next step is to explore the potential implementation of forecast-informed operations at two upstream reservoirs: Hell Hole Reservoir, operated by the Placer County Water Agency, and Union Valley Reservoir, operated by the Sacramento Municipal Utility District.

These efforts could create a network of reservoirs optimized for drought and flood using weather forecasts — a worthwhile investment to guard against the more intense droughts and storms that are already here because of climate change.